The production of oxygen and nitrogen from atmospheric air is a power intensive process. It is always desirable to reduce the power consumption of such processes. It is particularly true for large plants, when both oxygen and a large fraction of the nitrogen are demanded at pressures much greater than that of the atmosphere. Example of such an application are the Integrated Gasification Combined Cycle and the Integrated Gasification Humid Air Turbine electrical power generation systems. In these systems, high pressure oxygen is needed for gasification of a carbonaceous feedstock, e.g., coal, and high pressure nitrogen can be fed to the gas turbine power generation system to maximize power output, control NOx formation and/or increase its efficiency. The objective of the present invention is to reduce power consumption of cryogenic air separation plants providing products in such applications.
U.S. Pat. No. 5,257,504 proposed a dual reboiler cycle with the lower pressure column working at pressures significantly higher than that of the atmosphere. The dual reboiler cycle results in a significant power saving over a conventional Linde type double column system. This power saving for the dual reboiler cycle is due to the availability of a higher pressure nitrogen stream directly from the cold box. The dual reboiler cycle is suitable for cases in which all of the products of the air separation unit are delivered as products at pressures equal to or higher than those directly available from the cold box. When not all of the nitrogen is needed at such pressures, a stream of the nitrogen by-product has to be expanded to a lower pressure, typically at a low temperature. The expansion of a large gas flow with a low expansion ratio usually makes such a system inefficient.
On the other hand, a triple column cycle was introduced by Latimer for the high-pressure-air liquid plant (Chemical Engineering Progress, Vol. 63, No. 2, pp. 35-59, 1967). The triple column cycle was designed for complete oxygen recovery as liquid product and nearly complete argon recovery. The cycle has a feed air pressure of 140 psig (10.7 bara) or higher, since the top of the high pressure column is thermally integrated with the bottom end of the medium pressure column, and top end of the medium pressure column is, in turn, thermally integrated with the bottom end of the lower pressure column. In the cycle, oxygen-enriched liquid containing 25% oxygen from the bottom of the high pressure column is fed into the medium pressure column; and crude oxygen liquid bottoms of the medium pressure column containing 35% oxygen is fed to the low pressure column. The cycle is not designed to produce large fractions of feed air as nitrogen at pressures significantly higher than atmospheric. Almost all of the nitrogen is produced at extremely high purity and near ambient pressure from the top of the low pressure column. The high feed air pressure required for the cycle makes it inefficient for most applications.
There have also been attempts in the prior ad to improve power efficiency by vaporizing at least a portion of the bottoms liquid from the high pressure column by recirculated and boosted-in-pressure nitrogen. For example, in U.S. Pat. No. 5,080,703, a portion of the nitrogen from the low pressure column is boosted in pressure and condensed against a vaporizing portion of the reduced pressure bottoms liquid from the high pressure column of the double column system. U.S. Pat. No. 5,163,296 teaches the condensing of a high pressure nitrogen stream, which is the expander effluent, in the bottoms reboiler of the high pressure column of the double column system.